It would have been produced about 8 years before her death. Was she aware it existed? Did she have any thoughts on it? How would a blind and deaf person witness a play in that day and age?
Yes, Helen Keller was absolutely aware of The Miracle Worker. Patty Duke played her on Broadway in 1959, and the two met in 1961, shortly before Duke starred in the film version. There are several pictures from when the two of them met.
William Gibson, when writing about his work on the script, does not talk about Helen Keller. Instead, he attributes most of the inspiration to Anne Sullivan's autobiography, rather than interviews.
Did Helen ever see the play? She hadn't in October 1959, and indeed she made very few public appearances toward the end of her life, and did very little writing. Polly Thomson, who was Helen's assistant after Anne Sullivan's death, spent nearly 25 years with her but died in 1960 and Helen didn't have a regular assistant after that. My guess is that she did not ever attend a production or see the film, and if her thoughts on it were recorded, I have not seen them.
But other DeafBlind people certainly saw plays during that time. If Helen had, the story would've been relayed to her via the Rochester method, spelling letters into her palm, which was how she communicated most of her life. This would not be a word-for-word repetition, most likely, but a summary of what was being said and done onstage. She might also have gotten the information via the Tadoma method (tactile lipreading), which she also used.
The same would likely happen for other DeafBlind attendees who used tactile sign language, Tadoma, Rochester, or any other form of communication. An interpreter would have worked with them to convey what is happening, just as happens today when DeafBlind people attend theater and movies. It's also important to remember that both blindness and deafness are spectrums - a DeafBlind person may have tunnel vision, and so sitting toward the back of the theater might allow them to view more of the action. This would necessitate any dialogue relay to happen nearby - they wouldn't be able to see an interpreter at the front of the stage while also viewing the stage from afar.
More often, though, during the first half of the 20th century, Deaf people have put on their own plays and films, rather than attending hearing ones; these might be easier for DeafBlind people to attend, too. I have written about this before on the subreddit, I'm having trouble finding it but if someone can pull it up please do share the link.